Short-Lived Radionuclides in Meteorites: Constraints on Nebular Timescales for the Production of Solids

Variations in the abundances of short-lived radionuclides such as ²⁶Al (τ_1/2 ≈ 0.74 Ma) and ⁵³Mn (τ_1/2 ≈ 3.7 Ma) in meteoritic solids may be used to infer relative formation intervals of these solids in the nebula at precisions of less than 1 Ma. In a strict chronometric interpretation of the isotopic variations, whereby criteria such as spatial and temporal isotopic homogeneity and closed system isotopic evolution are met, solid formation occurred in the nebula for at least several million years. This is longer than some theoretical and astronomical estimates for the duration of the active nebula. The evidence for live ⁴¹Ca (τ_1/2 ≈ 0.10 Ma) in meteoritic inclusions further indicates that the onset of solid formation occurred quite early, perhaps within a few hundred thousand years after the onset of the collapse of the sun's parent molecular cloud. Failure of the chronometric interpretation may arise for a variety of reasons, including but not limited to, the late, inhomogeneous injection of material from a nearby stellar source or the local production of short-lived radionuclides by an energetic particle irradiation, e. g., from T Tauri (X-wind) or galactic cosmic ray sources. Although some isotopic evidence exists that the criteria required for a strict chronometric interpretation are not met by each of the short-lived chronometers, there is no compelling reason to shorten the interval of solid formation in the nebula to less than 1 Ma. This revised version was published online in June 2006 with corrections to the Cover Date.     

PLM,未来企业的数字化枢纽

挑战 航空航天与国防企业正面临如下挑战:如何在更长的产品生命周期内(30～50年或更久)同步地设计、制造和维修这些复杂产品,以及对拥有几千名或几万名员工的扩展型企业的业务和工作流程进行有效的管理.与此同时,"合同数据要求清单/转包合同数据要求清单"(CDRL/SDRL)、"国际武器贸易条例"(IRAR)等监管需求也在迅速升级.